Method for the determination of the average long-term power of digital modulated signal

ABSTRACT

The invention relates to a method for the determination of the average long-term power of a digital modulated input signal (S x ), comprising the following method steps: determination of the short-term power of a section of the input signal (S x ); creation of the data sequence (D x ) contained in the section (T 1;  T 2 ) of the input signal (S x ); creation of a reference signal (R 1 ) by modulation with the data sequence (D x ) contained in the section (T 1;  T 2 ) at a pre-set reference level (P 1 ) of the modulator ( 6 ), whereby the long-term power of the reference signal (R 1 ) at the reference level (P 1 ) is known; determination of the short-term power of the reference signal (R 1 ) within the section (T 1;  T 2 ); determination of a correction value (K) by comparison of the short-term power of the reference signal (R 1 ) with the long-term power of the reference signal at the reference level (P 1 ) and correction of the ¢ short-term power of the section (T 1;  T 2 ) of the input signal (S x ) with the correction value (K).

[0001] The invention relates to a method for determining the averagelong-term power of a digitally modulated signal.

[0002] A digitally modulated signal for mobile telephony, in particularfor GSM, usually consists of individual transmission blocks (bursts)which are modulated by an in-phase component I and a complementary-phasecomponent Q. The modulation is carried out, for example, as 8PSK.

[0003] When measuring the power of such a signal, the problem arisesthat the power measured within the measurement interval is dependent onthe modulation data. However, it is in fact the average long-term poweraveraged over a theoretically infinite time interval which is ofinterest. In order to find this long-term power, it has hitherto beencustomary to measure a very large number of transmission blocks (bursts)and to average the measured signal accordingly. However, this entailsrelatively long measurement times and is therefore disadvantageous.

[0004] It is therefore an object of the invention to provide a methodfor determining the average long-term power of a digitally modulatedsignal, which permits measurement in just one transmission block (burst)within a relatively short measurement interval.

[0005] The object is achieved by the features of claim 1.

[0006] The invention utilises the discovery that a short-termmeasurement within a relatively short time interval is sufficient fordetermining the long-term power if the data content within the measuredtime segment is determined and the measured power is corrected as afunction of this data content. According to the invention, it istherefore proposed to produce the data sequence contained in themeasured segment of the input signal, for example by demodulation, andto drive a modulator with a predetermined reference control factor byusing this data sequence, the average long-term power of the referencesignal corresponding to the reference control factor being known. If theshort-term power of the reference signal in relation to the data contentis determined in the time segment, then a correction value is obtainedwith which the measured short-term power of the input signal can becorrected.

[0007] The dependent claims contain advantageous refinements of theinvention.

[0008] The average long-term power of the reference signal is preferablydetermined by modulation with a statistical data sequence at thereference control factor. This determination of the long-term power atthe reference control factor only needs to be carried out once, and canthen be stored as a fixed value.

[0009] In the simplest case, the data sequence which is contained in thetime segment of the input signal is determined by demodulating the inputsignal. The GSM signal usually has a specific reference sequence with 26symbols in the middle of the transmission block (burst). The number ofdifferent reference sequences is limited. If the power measurement iscarried out in the vicinity of this reference sequence, then it ispossible to determine by comparison, optionally without demodulation,which reference sequence exists in the measured burst. The data contentof the reference sequence is then known.

[0010] Exemplary embodiments of the invention will be explained in moredetail below with reference to the drawing. In the drawing:

[0011]FIG. 1 shows a first exemplary embodiment of a block diagram forcarrying out the method according to the invention;

[0012]FIG. 2 shows an example of a transmission block (burst) and

[0013]FIG. 3 shows a second exemplary embodiment of a block diagram forcarrying out the method according to the invention.

[0014] Before discussing the exemplary embodiment which is representedin FIG. 1, an example of a transmission block (burst) in GSM format willfirstly be explained with reference to FIG. 2. The signal level isrepresented on a logarithmic scale as a function of the time, which isscaled in symbol intervals. The burst, consisting of a total of 147symbols, is divided into a first data sequence D1 a reference sequence Rand a second data sequence D2. Because of the IQ modulation, theamplitude of the signal is subject to relatively large fluctuations. Inthe example which is represented, these fluctuations amount toapproximately 15 dB.

[0015] Whereas the data in the two data sequences D1 and D2 isarbitrary, a reference sequence R, which is selected from a limited setof reference sequences, is carried over 26 symbols in the middle of theburst.

[0016] The problem when carrying out a power measurement on the signalrepresented in FIG. 2 is that, because of the amplitude fluctuations dueto the modulation, the power measured within a time window that lieswithin the burst is dependent on the data content which is transmittedduring this time interval. For the measurement, however, it is theaverage long-term power, independent of the data content, which is ofinterest. In order to find this, it has hitherto been customary tomeasure and then average the power within a very large number of bursts.Since the measurement accuracy is proportional to the square root of theindividual measurements, a very large number of individual measurementsare necessary in order to achieve a satisfactory measurement accuracy.This entails a relatively long overall measurement time.

[0017] According to the invention, it is therefore proposed to weightthe short-term power measured within a time segment T1 or T2 with acorrection factor that is dependent on the data content with which thesignal has been modulated during the time segment.

[0018] The input signal S_(x) to be measured is delivered to an inputterminal 2 of a measuring instrument 1 which is schematicallyrepresented in FIG. 1. The input signal S_(x) is a digital samplesignal. In a first averaging unit 3, the mean value of the input signalS_(x) to be measured is formed within the measured time segment. Themeasured time segment is indicated by way of example in FIG. 2 as T1 fora measurement within the data sequence D₁ and as T2 for a measurementwithin the reference sequence R. The mean power is calculated by takingthe square of all the sample values, adding the squared sample valuesand dividing by the number of sample values (MS=mean square). If it isnot the mean power but the mean amplitude which is of interest, thesquare root may also be taken (RMS=root mean square). The output of thefirst averaging unit 3 is delivered to the +terminal of a firstsubtraction unit 4.

[0019] In order to find the data content within the measured timesegment T1 or T2, the input signal S_(x) is delivered to a demodulator5. The data sequence D_(x) modulating the input signal S_(x) within thesegment T1 or T2 is provided at the output of the demodulator 5, and itis delivered to a first modulator 6. The modulator 6 is operated using aconstant, predetermined reference control factor P₁. This referencecontrol factor P₁ is known. In order to determine the power of thereference signal R₁ which is modulated using the modulator 6, a secondaveraging unit 7 is used which is connected to the output of the firstmodulator 6. The power of the reference signal R1 is modulated bysquaring and averaging the squared amplitudes within the segment T1 orT2 (MS=mean square).

[0020] The output of the second averaging unit 7 is delivered to the+terminal of a second subtraction unit 8. The second subtraction unit 8compares the data-dependent short-term power of the reference signal R₁with a data-independent long-term power of the reference signal R₁. Tothat end, a reference signal R₁′ is produced using statistical data Dsover a very long measurement interval. This is schematically representedin FIG. 1 by the fact that the output of a statistical data source 9 isdelivered to a second modulator 10, which is operated with a controlfactor P2 that corresponds exactly to the control factor Pi of the firstmodulator 6. The output of the second modulator 10 is delivered to athird averaging unit 11, which in turn squares the sample values, takesthe sum and divides by the number of sample values (MS=mean square), sothat the mean long-term power of the reference signal R₁′ modulated withthe statistical data Ds is provided at the output of the third averagingunit 11. The thus found mean power of the reference signal at thecontrol factor P₁=P₂ is constant, so that the method steps describedwith reference to the block diagram elements 9 to 11 only need to becarried out once, and the resulting constant value can be stored.

[0021] The output of the third averaging unit 11 is delivered to the-terminal of the second subtraction unit 8. At the output of the secondsubtraction unit 8, a correction value K is obtained which is positivewhen, because of the data modulation, the short-term power measured forthe reference level R₁ is greater than the long-term power to beexpected with statistical data D_(s), and which is negative when,because of the data modulation, the measurement interval involves alower power than is to be expected for a long-term measurement withstatistical data D_(s). It should also be pointed out that the averagingunits 7 and 11 form the output level on a logarithmic scale, which isindicated by the symbol “dB”. For this reason, subtraction rather thandivision of the output signals is required. If the averaging units 11and 7 provide output levels on a linear scale, then the subtraction unit8 must be replaced by a division unit. The correction value K islikewise scaled logarithmically, and it is delivered to the -terminal ofthe first subtraction unit 4, the correction value K being subtractedfrom the power measured within the segment T1 or T2. If the data contentgives rise to a positive deviation from the long-term power to beexpected for the modulation with statistical data DS, then the measuredshort-term power will correspondingly be corrected downwards.Conversely, the measured short-term power will be corrected upwards if,because of the data sequence D_(x). contained in the measurementinterval, a negative deviation is encountered from the long-term powerfor modulation with statistical data D_(s).

[0022] The method according to the invention is represented as a blockdiagram in FIG. 1. Nevertheless, the method according to the inventionneed not be carried out using circuit technology (as hardware), but mayalso be fully implemented by program steps (as software). The measuredand corrected power is provided at the output 12.

[0023] In particular, the exemplary embodiment represented in FIG. 1 issuitable for a measurement interval T1 within the two data sequences D₁and D₂, when nothing is known about the data content within themeasurement interval. If, however, measurement is carried out within thereference sequence R in the middle of the burst, which is indicated inFIG. 2 by the segment T2, then demodulation is not necessarily requiredif the reference sequence R is selected from a limited number ofpossible reference sequences. In that case, it is possible to comparethe input signal within the measurement interval with the possiblereference signals, without demodulation being required. Such anexemplary embodiment is represented in FIG. 3 as a schematic blockdiagram. Elements which have already been described with reference toFIG. 1 are provided with matching references, so that a repeatdescription is in this respect unnecessary.

[0024] The input signal S_(x) within the measurement interval iscompared with possible signal waveforms S₁ to S₁₁, for example bycorrelation. The signal S₁ to S₁₁ which has the best match with theinput signal S_(x) is selected, and the modulation data D₁ to D₁₁corresponding to this signal is delivered to the modulator 6. Undercertain circumstances, such a comparison may be carried out more rapidlythan demodulation, and demodulation errors are avoided.

1. Method for determining the average long-term power of a digitallymodulated input signal (S_(x)) with the following method steps:determining the short-term power of a segment (T1; T2) of the inputsignal (S_(x)), producing the data sequence (D_(x)) which is containedin the segment (T1; T2) and with which the input signal (S_(x)) ismodulated, producing a reference signal (R₁) by modulation with the datasequence (D_(x)) which is contained in the segment (T1; T2) at apredetermined reference control factor (P₁) of the modulator (6), thelong-term power of the reference signal (R₁) corresponding to thereference control factor (P₁) being known, determining the short-termpower of the reference signal (R₁) within the segment (T1; T2),determining a correction value (K) by comparing the short-term power ofthe reference signal (R₁) with the long-term power of the referencesignal corresponding to the reference control factor (P₁), andcorrecting the short-term power of the segment (T1; T2) of the inputsignal (S_(x)) by using the correction value (K), in order to obtain thelong-term power of the input signal (S_(x)).
 2. Method according toclaim 1, characterised in that the long-term power of the referencesignal (R₁′) is formed by modulation with a statistical data sequence(D_(s)) at the reference control factor (P₁) and averaging over anextended time period.
 3. Method according to claim 2, characterised inthat the long-term power of the reference signal (R₁′) is formed onceand is stored in a memory.
 4. Method according to one of claims 1 to 3,characterised in that the data sequence (D_(x)) which is contained inthe segment of the input signal (S_(x)) is produced by demodulating theinput signal (S_(x)).
 5. Method according to one of claims 1 to 3,characterised in that data sequence (D_(x)) which is contained in thesegment of the input signal (S_(x)) is produced by comparing the inputsignal (S_(x)) with comparison signals (S₁ . . . S₁₁), which aremodulated with different data sequences (D₁. . . D₁₁).